1. Field of the Invention
The present invention relates to a substrate for a semiconductor device having a resistor film capable of adjusting a resistance value by laser trimming technique and the like and more particularly to an improvement in the resistor film.
2. Description of the Background Art
There is shown in FIGS. 9 and 10 a trimmed printed resistor portion for a conventional substrate for a hybrid integrated circuit. The substrate for the hybrid integrated circuit board includes a body 11 made of alumina. On the surface of the body 11, there are provided a pair of spaced strip-like conductor films 12A, 12B in parallel. A resistor film 13 is a thick-film resistor formed by printing and is substantially square in plan view. One side of the resistor film 13 is joined to the strip-like conductor film 12A, and the opposite side thereof is joined to the strip-like conductor film 12B. A protective coat 14, which is an overcoat glass, covers the pair of strip-like conductor films 12A, 12B and the resistor film 13.
In such a conventional structure, the resistance value R of the resistor film 13 is determined by .rho..multidot.{l/(w.multidot.t)} where t, w, l and .rho. are the thickness, width, length and resistivity of the resistor film 13, respectively.
The resistor film 13 shown in FIGS. 9 and 10 is formed with an L-shaped trace 15 produced by the laser trimming technique. The trace 15 extends widthwise from a side end of the resistor film 13 and bends longitudinally at a predetermined position. In the conventional substrate for the semiconductor device, the resistance value of the resistor film 13 is desirably adjusted by forming the trace 15.
However, when the resistance value is adjusted by the laser trimming technique, not only the resistor film 13 but also the protective coat 14 for covering the resistor film 13 are removed. This causes the resistor film 13 to be exposed to an external atmosphere along the trace 15, as is apparent from FIG. 10. There arises a problem that the resistance value of the resistor film 13 is changed with time by moisture included in the external atmosphere.
It is not a customary practice to coat the resistor film exposed along the trace 15 again with the protective coat after the adjustment of the resistance value. If the resistor film is coated again with the protective coat, increase in the number of processes is accompanied by the rise in cost.